Reference: Duroos wa Fataawa al-Hajj – Volume 1, Page 14, No.13
Question: What is the ‘as-salaah al-wustaa’?
Response: According to the correct opinion, ‘as-salaah al-wustaa’ is the ‘asr prayer; this is based upon his ﷺ statement [on] the day of Ahzaab:
«They occupied us away from as-salaah al-wustaa, the ‘asr prayer», transmitted by al-Bukhaaree (2931) and Muslim (628)